1. Field of the Invention
The present invention relates to a stacked structure of a spiral inductor, and more particularly, to a stacked structure for improving quality factor by making use of metal layers of a crossover region as shunt windings.
2. Description of the Prior Art
As IC manufacturing processes are developing with a trend for system on chip (SOC) design, passive elements such as integrated inductors or integrated transformers have been widely integrated in high-frequency integrated circuits. Since IC manufacturing processes usually adopt a structure with Silicon-based substrates, substrate loss and metal loss of the integrated inductors/integrated transformers will affect their quality factor. And more particularly, metal loss is the main cause that most immediately affects the inductor properties.
Recently common integrated inductors consist of symmetric inductors and non-symmetric inductors. Most of them adopt the top metal layer (or the top two metal layers) as a winding of the inductor, and use the next metal layer as well as vias as a crossover region. Such inductor has the following disadvantages, that is: its parasitic resistor will be restricted by the single metal layer, a conductivity of the crossover region, a number of the vias, and the resistance of the vias. In order to improve the quality factor of the inductor, increasing widths of the winding is required. However, this approach will occupy more chip area. Hence, a structure with multiple metal layers in parallel is proposed, such as in the US patent with patent No. 2008/0074229 and the U.S. Pat. No. 6,664,882 proposed in the prior art, such that the inductor can have a lower series resistance than a traditional single-layer inductor so as to improve its quality factor. However, no matter the structure with a single metal layer or the structure with multiple metal layers in parallel are concerned, the metal layers of their crossover regions of the inductor are different from the metal layers of the windings. For this reason, the quality factor of the inductor cannot be optimized.